Various graphic printers have been described wherein a sheet of paper is positioned on a rotating drum and maintained in place by means of a vacuum created within the drum. A controlled ink stream is directed at the paper, printing a desired pattern printed thereon. U.S. Pat. application No. 606,954, filed Aug. 22, 1975 by Hauser et al. and entitled "Paper Handling in Graphic Printer" having a common assignee with this application, describes such an arrangement. As shown in this application, the paper is carried on an apertured platen drum which is rotated past a slowly moving print head so as to raster scan from top to bottom and from left to right over the surface of the paper. To facilitate insertion and removal of the paper, a shroud is placed very close to the cylindrical surface of the drum and during insertion, guides the paper into close proximity with the vacuum platen. An exit door is provided in the shroud, at a convenient location, to permit the paper to exit from the drum once the vacuum has been released. The size of the drum is made such that its circumference is slightly less than the length of the paper so as to provide overlap of the top and bottom edges of the paper. A light beam, directed tangential to the platen, is interrupted by the flap during each revolution of the drum which condition is sensed by a photocell. Such a flap sensor is described in U.S. Pat. application Ser. No. 606,959 filed Aug. 22, 1975 by Sokolowski and entitled "Paper Overlap Edge Sensor" having a common assignee with this application.
A particular requirement of the previously described arrangement is that the paper should be securely positioned upon the drum before printing is started so as to maintain line registration throughout printing. It will be appreciated that should slippage occur between the paper and the drum during printing, which generally occurs as a result of print initiation before proper paper positioning is obtained, the resulting print will be blurred and unacceptable. Further, the paper sensing circuitry should be such as to ignore false paper conditions which might occur due to mishandling of the paper such as partially inserting and then removing the paper from the drum or holding one end of the paper while allowing the remaining end to rest on the drum. Should such situations be interpreted as valid paper conditions by the printer, the ink jet would be directed at the drum resulting in ink being drawn into the vacuum system and deposited throughout the apparatus which situation, it will be appreciated, is extremely undesirable. Additionally, not only should the paper sensing circuit be immune to operator interference, but it should also correctly reference the start of message to the top paper edge. Further, although the initial start of the print should be referenced to the top edge of the paper, continued referencing of each raster scan to the paper edge can result in copy distortion due to paper edge and/or flap sensing variations. Thus, continued start of print referencing to the flap sensor produces a copy wherein the print lines exhibit a wave effect depending on system interpretation of the paper edge. The illustrated embodiment references the start of copy to the top of the paper thereby accurately centering the print on the paper; however, subsequent raster scans are referenced to the position of the drum thus avoiding line registration errors introduced by variations in the paper edge.